Activated carbon is a form of carbon processed to create in it a large number of small and interconnected pores that increase the surface area available for adsorption or for chemical reactions. One exemplary use of activated carbon is removing, through adsorption, contaminant matter from water, other liquids, gases, earth and soil, etc. Activated carbon is usually produced from carbon rich structured biomass or coal material as precursors.
Carbon activation processes predominantly make use of two gasifying agents, carbon dioxide (CO2) and water (H2O), either in combination or alone. These reactants remove carbon atoms from the structure of the carbonaceous precursor material, in a way which increases the porosity of the material left behind. This happens according to the following two reactions:C(s)+CO2(g)→2CO(g)C(s)+H2O(g)→CO(g)+H2(g)
Where no chemical activation agents are used, activated carbon production proceeds in a two-step process. In the first step (carbonisation), a carbonaceous precursor material is heated in the absence of oxygen to remove volatile organic compounds. In the second step (activation), the charred material from the first step is further heated in the absence of oxygen. For all prior art commercial production technologies, activation of carbon is carried out at high temperatures (800-1100° C.) over process times of hours. The duration of the activation step is chosen in line with desired targets for product yield and the degree of activation.
Production technologies and processes for activated carbon fall into three main types: rotary kilns, multiple hearth furnaces, and fluidized beds. The carbonisation and activation steps can be carried out in any combination of these reactor types according to product specification requirements.
In rotary kiln and multiple hearth furnace reactors, the reaction rates are slower and the equipment is larger and more expensive than for fluidised beds. Both technologies involve a complex assembly of moving parts.
Fluidised bed reactors create a bed of solid carbonaceous precursor particles that is supported by a (mostly upward) stream of activating gases so that the mixture of solid particles and gases behaves as a pseudo-fluid. Compared to the other commercial technologies for producing activated carbon, the conventional fluidised bed reactor provides much higher surface contact area and higher velocities between solid particles and process gas, exposes a larger proportion of the particles to the activating gas flow and promotes particle collisions with each other and with the wall of a reactor in which the fluidised bed is formed. All these effects promote improved heat and mass transfer rates and a reduction in required particulate residence times for activation.
However, the use of conventional fluidised beds for producing activated carbons suffers a fundamental limitation in that such beds can only operate with superficial gas velocities (u0) (typically defined as the volumetric rate of gas flow divided by cross sectional area of the gas path) between certain defined limits. Firstly, the gas velocity must be higher than the velocity at which incipient fluidisation occurs (minimum fluidisation velocity, uminf). Ratios of u0/uminf up to around 10 are typically used in practice. Secondly, the superficial gas velocity must be below the velocity (referred to as the terminal velocity ut of a particle) at which the solid particles start to be entrained in the upwards gas flow and escape from the top of the vessel in which the fluidised bed is formed (elutriation, carryover). Gas velocities in excess of the terminal velocity may be used in fluidised beds, but this necessitates the use of external apparatus, for example a cyclone separator, to capture escaped particles and recirculate them through the reactor, which adds further plant complexity. The limits uminf and ut therefore generally define a practical operating window for a feed material with a given particle size distribution; this window is generally smaller for larger particles.
The practical limits on gas velocity and reaction temperature mean that there is limited scope to obtain significantly improved reaction rates or efficiency using conventional approaches.
As such, there remains an ongoing need to improve the efficiency and reduce the cost of the production of activated carbon. Moreover, while biomass materials can be and are routinely used as precursors for such production, coal and charcoal based feed material remains comparatively inexpensive, and there is accordingly a particular need to improve techniques for activation of such feed materials. Importantly, unlike biomass materials, there are no supply chain constraints on coal and charcoal feedstocks and they are widely available. This better supports the building of commercially sustainable business models.